Sensors have been used to measure flow rates in various medical, process, and industrial applications, ranging from portable ventilators supplying anesthetizing agents to large-scale processing plants in a chemical plant. In these applications, flow control is an inherent aspect of proper operation, which is achieved in part by using flow sensors to measure the flow rate of a fluid within the flow system. In many flow systems, e.g., fuel cell flow systems containing a binary mixture of methanol and water, the chemical composition of the fluid may change frequently.
A flow system is often required to flow more than one fluid having different chemical and thermo physical properties. For example, in a semiconductor processing system that passes a nitrogen-based gas, the nitrogen-based gas may at times be replaced by a hydrogen-based or helium-based gas, depending on the needs of the process; or in a natural gas metering system, the composition of the natural gas may change due to non-uniform concentration profiles of the gas.
Fluid flow sensors are thus important in a variety of applications. It is often necessary to determine the composition of a fluid utilizing a liquid or fluid flow sensor. One method for determining the composition of the fluid is to measure its thermal conductivity and compare the resulting value to a standard value. Measurements can be obtained by measuring power transferred from a heater to the fluid.
Current approaches for fabricating liquid flow sensors for very low flow applications are constrained by methods of coupling flow to the flow sensor while maintaining a minimal system volume. Coupling between the sensing element and an isolated flow channel has proved difficult to produce. Earlier processes involved the use of a nickel sacrificial layer, which often oxidized, resulting in an undependable final release. It is believed that such problems can be overcome through the introduction of a flow sensor structure and fabrication technique in which the flow channel is built directly on the detector wafer and thereby couples the heat transfer directly to the liquid flow sensor. Such improvements are discussed in, greater detail herein.